In the field of radar, radio and other signal transmission applications, it is often desirable to convert a signal from one frequency to a lower frequency. Typically, this has been done using multiple local oscillators with associated filters, mixers, intermediate frequency amplifiers and phase-locked loop circuitry. Such circuits consume power, are inherently lossy, and can emit spurious unwanted harmonic signals. These unwanted emissions can, with appropriate equipment, be detected and hence reduce the stealth capabilities of applications using such circuits. Additionally, a number of components associated with these local oscillators cannot readily be implemented in integrated circuits, requiring off-chip circuit elements such as crystals and inductors.
When utilizing field portable power sources, it is desirable that power consumption be minimized, and power be utilized efficiently. Reducing the circuit element count in a circuit can reduce power consumption, but utilizing traditional local oscillators and mixers for down-conversion has a practical limitation in traditional local oscillator-based designs.
A key element in enabling direct down-conversion of signals is a track and hold (T&H) amplifier to provide a steady sampled input to an analog to digital (A/D) converter. The current state of the art is capable of making A/D converters with resolutions for electronic warfare applications (EW) at eight bits at 1.5 GHz. Because of finite transition bands required by analog anti-aliasing filters, Nyquist criterion sampling (requiring a sampling rate of at least twice the frequency of the highest frequency in the signal to be sampled to avoid aliasing) limits signal bandwidth to about 500 MHz.
As a result, radio frequency (RF) components have still been required for amplification, frequency conversion and filtering. Such components generally consist of multiple tunable local oscillators mixers, filters and intermediate frequency (IF) amplifiers.
What is needed is an efficient (i.e. low circuit element count and power consumption) high speed (greater than 1 GHz) track and hold amplifier to be able to implement down-conversion, while minimizing spurious emissions to improve the stealth characteristics of the application.